It is known in the art of injection molding to simultaneously or sequentially inject two melt streams of moldable material into a mold cavity using a single hot runner injection molding nozzle, which is commonly referred to as coinjection. A conventional manner of controlling the flow of two or more melt streams through the nozzle and into a mold gate and subsequently the cavity has been provided by rotating a valve pin member of the nozzle to align different melt channels or by axially reciprocating a valve pin member and one or more valve sleeve members, which surround the valve pin member, of the nozzle between open and closed positions. Although many systems have been developed utilizing a valve pin member and a valve sleeve member that are axially reciprocated to provide simultaneous or sequential injection of two or more melt streams, such arrangements are not without their deficiencies, such as inaccuracies in reciprocating movement and difficulties in keeping the melt streams adequately separated, as well as adding complexity to the manufacture, assembly, and operation of the hot half of the injection molding systems. Another deficiency in such systems is that it is difficult to align a valve sleeve member and/or a valve pin member with the mold gate, such aligning being important for improving injection technique and reducing gate wear. In a multi cavity hot runner injection molding system creating consistent parts from cavity to cavity has long been a challenge.
In some conventional coinjection systems, such as those described in U.S. Pat. No. 3,947,177, U.S. Pat. No. 6,596,213, and U.S. Pat. No. 7,517,480, a volume of a core layer melt stream to a volume of inner and outer layer melt streams may be controlled at the injection units by setting a shot size and injection velocity of each melt stream provided thereby. In other conventional coinjection systems, such as U.S. Pat. No. 5,914,138, viscosity of the various melt streams is controlled to affect a volume of the core layer melt stream relative to the volumes of the inner and outer layer melt streams entering a given mold cavity.
A volume of a core material for producing a core layer of a molded article is particularly relevant in ‘barrier’ coinjection molding applications, wherein the core layer of a barrier material is a tiny fraction of the total combined melt streams entering a given mold cavity, and in ‘filler’ coinjection molding applications, wherein the core layer of a filler material is a large portion of the total combined melt streams entering a given mold cavity. In both ‘barrier’ and ‘filler’ coinjection molding applications providing precise equal amounts of the core material to each individual mold cavity is critical in order to ensure consistent molded parts across the mold. Often such control of a ratio of the core layer material to the inner and outer layer material occurs at the utmost upstream end of the hot runner system, i.e., at the machine barrel that supplies each material, which cannot take into account shear history differences/imbalances that may occur by the time a particular melt stream reaches a mold cavity at the downstream end of the hot runner system. During a given injection cycle, shear history imbalances may result in some mold cavities receiving too much core material and some mold cavities receiving too little core material. Since the precise amount of core layer material is critical to ensuring quality coinjected molded articles, even a slight imbalance between mold cavities can have a large impact.
The aforementioned problems may be exacerbated in cases where a molded article requires different core layer thicknesses within a single molded article, such as in a molded closure having a thinner core layer in a threaded region and a thicker core layer in an end region. More particularly, if a throttling or other adjustment of the core layer material is done at the upstream end of the molding system, for instance in the machine, it is often more difficult to control the exact location in a molded article where a core layer thickness will transition in each mold cavity.
Embodiments hereof address at least some of the problems identified in the coinjection applications described above by providing a mechanism that throttles or adjusts a core layer material flow proximate a downstream end of the hot runner system, and more specifically throttles or adjusts the core layer material flow within a nozzle tip thereof, to allow for more precise control of a volume of core layer material to a volume of material used to form inner and outer layers of a molded article within each individual cavity. Such control proximate a mold gate of a mold cavity may result in a greater overall consistency between all of the cavities of the coinjection molding system.